Method of brightening high density cellulose pulps with a reducing bleaching salt at high speeds



United States Patent METHOD OF BRIGHIENING HIGH DENSITY CEL- LULOSE PULPS WITH A REDUCING BLEACH- ING SALT AT HIGH SPEEDS Robert L. McEwen, Williamsville, and Charles W. Ra-

No Drawing. Application December 2, 1954 Serial No. 472,768

2 Claims. (Cl. 162-82) This invention pertains to a method of bleaching cellulosic pulp, pulp mixtures, and paper furnishes with reducing agents considered as derived from those oXy acids of sulfur wherein sulfur has a valence of less than '6, and more particularly to a method of bleaching such pulp at high density with such reducing agents.

The use of reducing agents to bleach cellulosic fibers, such as cotton, and to brighten cellulosic pulps such as are used in paper making, is well known in the art. A number, of processes have become known more recently which employ reducing agents particularly for the bleaching of mechanical or groundwood pulps and also for the bleaching of chemical pulps. Treatment of pulps with reducing agents represents an essentially very simple and generally satisfactory bleaching method. However, certain requirements must be met and operating conditions chosen it efficient pulp bleaching with reducing agents is to be achieved.

The prior art has shown that in the bleaching of pulp with reducing agents, pH control is of importance and that, particularly, if bleaching rates are increased by Working at higher temperatures, careful control and maintenance of pH in a critical range is necessary.

The prior art has also indicated that when using reducing agents for bleaching, the ratio between pulp fibers and water is very important. It was found that the bleaching efliciency of reducing agents tends to tie crease with a decrease in the ratio of pulp to water. In other words, the higher the pulp density, the lower the bleaching efficiency. On the other hand, bleaching at very low pulp densities is undesirable because here,

again, high dilution with water tends to reduce bleach ing efliciency. As a result, a relatively narrow range of pulp densities, and more particularly the range between about 2 /2% and 5%, has been recommended as the optimum range for obtaining satisfactory bleaching re- -'sults'when brightening pulps with reducing agents.

Pulp bleaching at densities below 5%, even if efficient chemical-1y, is inefficient from an overall operating viewpoint. Low density bleaching inevitably requires large storage and retention equipment to handle the huge volume of low density pulp. A factor contributing to low production rates, when bleaching at low pulp consistencies, is the relatively prolonged bleaching time required, which time is usually of the order of a few hours. On the other hand, the steam requirements at low densities are high because, even though operating temperatures are usually of the order of 100 to 140 F. only, it becomes necessary to heat an excessively large volume of water together with the relatively small amount of pulp fiber assembled in the water.

Techniques have been recommended in an attempt to overcome some of these disadvantages. It has been suggested that the reducing agent should be incorporated in a -low density pulp slurry, which should then be fed to 'fl..=paper machine without intermediate storage. Here ICC the bleaching action was supposed to take place, in part at least, during the paper or sheet-forming operation. In such proposed processes, which would avoid some of the above mentioned disadvantages, difiiculties arose because of the corrosive effect of the reaction products on dewatering equipment and particularly on the paper making wire or sheet making equipment at the low pulp density employed. v

All in all, the use of reducing agents in the bleaching of pulp was thought to require operation at low density and moderate temperatue, conditions under which operating efficiency of the bleach plant, as contrasted to chemical efiiciency of the bleaching reaction, is relatively low.

We have now found, in contrast to the teachings of the prior art, that the bleaching of cellulosic pulps or mixtures with reducing agents is possible at high pulp densities and at high speeds. No lengthy retention times are involved, thus avoiding the need for high pulp inventory in process, no special pulp handling equipment is necessary and the process is easily adaptable to a variety of pulps, furnishes, and plant conditions.

The process of this invention may be used for the bleaching of cellulosic fibers in general, and particularly to the bleaching of cellulosic pulps or paper furnishes comprising groundwood or mechanical pulps as well as chemically treated pulps such as semi-chemical, sulfite, soda and kraft pulps. The process is applicable to individual pulps, pulp mixtures, or to paper furnishes which may or may not contain additions such as clay, rosin sizes, alum, wet strengh resins, etc. Moreover, the pulp may be treated in a variety of physical forms, such as sheets, crumbs, paper and paperboard provided only that the pulp be of high density and that substantial amounts of moisture be removed during bleaching. These various pulp types and paper furnishes and forms will hereafter be briefly called pulps.

The process of this inventionemploys reducing agents such as those considered as derived from those oxy acids of sulfur wherein the valence of sulfur is less than 6 although, in specific cases, specific reducing agents from this class may be preferred for reasons of cost, availability, etc. Examples of reducing agents of this class suitable for use in this process are sodium hydrosulfite, zinc hydrosulfite, zinc sulfoXylate formaldehyde, sodium sulfoxylate formaldehyde and sodium bisulfite or their acids. The relatively water soluble salts generally of these oxy acids of sulfur are suitable as the bleachant but, by reasonof ease of preparation, price and availability, those salts just mentioned are preferred.

In accordance with the principles of this invention the reducing agent is applied to high density pulp in such a manner that only restricted quantities of reducing agent solution are needed. After the required amount of .reducing agent has been incorporated into the pulp, a major portion of the moisture in the pulp is removed. The bleaching action proper, under these conditions, takes place very rapidly and may be thought to take place during the brief period of time required to remove excess moisture from the pulp.

Removal of excess moisture from the pulp may be accomplished in any convenient manner as simply by exposing the pulp to a current of air. For obvious reasons, heating will ordinarily be resorted to for the purpose of removing excess moisture but it is to be understood that in this process, heat, it applied, serves only to increase rate of moisture removal and is not required for the bleaching reaction proper to take place.

This invention is based on the discovery that commercially satisfactory bleaching effects are obtained on pulp, if relatively minor amounts of a reducing agentof the type mentioned are incorporated into the-pulp at'high pulp densities above. about 20%, and excesswater re- 2,912,297 3 4 moved immediately thereafter from the pulp. The ways ness of 25.9 GE units. In addition, a commercially and means by which the reducing agents are incorporated and the ways and means by which excess water is removed may be chosen in an arbitrary manner and depend on existing equipment and conditions and do not form a critical part of this invention.

For example, pulp in form of crumbs may be carried on a conveyor and the reducing bleachant incorporated in the pulp crumbs by spraying an aqueous solution of the reducing bleachant on to the pulp whereupon the treated pulp is passed through a drying zone for the sole purpose of removing excess water. Or, a pulp sheet, after formation on a papermaking machine and at a density of at least may be treated with an aqueous solution of the reducing agent close to or immediately prior to the point on the machine where the moist sheet enters the drying section of the paper machine. Any other techniques which permit incorporating a solution of the reducing agent into the moist pulp and subsequent removal of excess water from the pulp without removal of the bleachant may be used.

To obtain the advantage connected with the instant high speed, high density pulp bleaching operations, the pulp, before incorporation of the bleaching agent, should have a moisture free density of at least 20% or from approximately 20 to 60% more or less. Moreover, the step of water removal should be carried out in such a manner that after its completion, the pulp will have a moisture free density of about 65 to 95%. Furthermore, to obtain the maximum operational advantages from this method, the ways and means of removing excess water should permit its performance within a short period of time on the order of a few minutes, although lower rates of moisture elimination are within scope of this invention thus providing for progressive concentration of the bleachant.

The following examples will serve to illustrate the principles of this invention. In some of these examples, a buffering agent or acidic material is used together with a reducing agent to adjust pulp pH, i.e., the pH of the pulp environment to greater than pH 3 and less than pH 10.

Example 1 A pulp comprising sulfite pulp was impregnated at 50% consistency with an aqueous solution of such concentration to incorporate 0.5% sodium hydrosulfite and 1% sodium dihydrogen phosphate into the pulp, based on the dry weight of the pulp. After treatment, the pulp was dried in a current of air at room temperature to 95% consistency. After moisture removal, the brightness of the pulp was 63.3 GE units, while the brightness of the untreated pulp Was 56.1 GE units.

Example 2 Another sample of the same pulp was treated as described in Example 1 but excess moisture was removed by passing the pulp over a steam heated cylinder. After this treatment, the pulp dried to a consistency of 95% showed a brightness of 6l.l GE units whereas the untreated pulp possessed a brightness of 56.1 GE units.

Example 3 A pulp comprising groundwood pulp was treated as in Example 1 by incorporating 1% sodium hydrosulfite and 2% sodium tripolyphosphate. After removing excess moisture by passing the pulp over a steam heated cylinder whereby the pulp was dried to 95% consistency, the pulp showed a brightness of 64.6 GE units, whereas the untreated pulp had a brightness of 58.3 GE units.

Example 4 A pulp comprising kraft pulp was treated as described in Example 3. The treated pulp showed a brightness of 32.8 GB units, whereas the untreated pulp had a brightinteresting shade change resulted.

Example 5 A pulp comprising soda pulp was treated at 50% consistency with an aqueous solution so as to incorporate on a dry basis, 0.2% sodium sulfoxylate formaldehyde and 1% sodium dihydrogen phosphate. After moisture removal to consistency, a brightness of 48.7 GB units was obtained in contrast to a brightness of 43.3 GE units of the untreated pulp.

Example 7 A pulp comprising bleached soda pulp was treated as described in Example 6. The treated pulp had a brightness of 79.1 GE units, whereas the untreated pulp had a brightness of 77.8 GE units.

Example 8 A pulp comprising neutral sulfite semi-chemical pulp was treated as described in Example 3 and after treatment showed a brightness of 53.4 GE units as compared to a brightness of 42.3 GE units before treatment.

Example 9 A pulp comprising a mixture of about 50% groundwood pulp previously bleached with hydrogen peroxide and 50% bleached sulfite pulp, was treated as described in Example 3. The treatment produced a brightness of 77.1 GB units compared to a brightness of 72.2 GE units before treatment.

Example 10 A pulp comprising a mixture of about 50% groundwood pulp previously bleached with zinc hydrosulfite and 50% bleached sulfite pulp was treated as described in Example 3. The treatment produced a brightness of 73.5 GE units as compared to a brightness of 69.3 GE units before treatment.

Example 11 A pulp comprising a mixture of about 50% groundwood pulp and 50% sulfite pulp was treated as described in Example 3. The treatment gave a brightness of 70.0 GE units as compared to a brightness of 64.7 GE units before treatment.

Example 12 A pulp comprising a mixture of about 50% groundwood pulp and 50% sulfite pulp was prepared and samples of that pulp were treated as described in Example 3, using various reducing agents.

Treatment G.E.

Brightness (a) Untreated 57.5 (b) {1% sodium hydrosulfite 2% sodium tripolyphosphateu l 5 (c) 1% zinc hydrosulfite "c.

1% dlsodium phosphate 6L5 (d) 1% zinc sulfoxylate formaldehyde .M.

1% sodium dihydrogcn phosphate l 2 (c) 0.1% sodium sulioxylate lormaldehyde 1% sodium dihydrogen phosphate 1 l (f) 1% sodium sulfoxylate formaldehyde 62.1 (g) 1% zinc sulfoxylate formaldehyde 62. 7

Example 13 Pulp samples were prepared by pressing to a density of 40% and an aqueous solution of bleaching agent was applied to these pulp samples by spraying in such a manner that on a dry basis, 1% sodium hydrosulfite, 1% sodium tripolyphosphate and 1% sodium dihydrogen phosphate were incorporated in the pulp. Thereafter moisture was removed from the samples by hot air drying to a density of 75%. Thereafter the samples were repulped and sheets formed from the resulting pulp slurry.

A pulp comprising kraft pulp treated in this manner showed a brightness of 32.5 GE units and compared to a brightness of 29.4 GE units before treatment, and a pulp comprising sulfite pulp showed a brightness of 61.1 GB units compared to a brightness of 58.9 GB units before treatment.

Example 14 A pulp comprising about 50% groundwood pulp and 50% sulfite pulp was treated as described in Example 3, incorporating 1% of sodium bisulfite into the pulp on a dry pulp basis. The treatment produced a brightness of 61.0 GB units compared to a brightness of 57.2 GB units before treatment.

Example 15 A pulp comprising a mixture of 50% groundwood and 50% sulfite pulp was impregnated at 25% consistency with an aqueous solution of such concentration to incorporate 1.0% zinc sulfoxylate formaldehyde into the pulp, based on the dry weight of the pulp. After treatment, the pulp was dried in a current of warm air to 95% consistency. After moisture removal, the brightness of the pulp was 66.6 GB units, while the brightness of the untreated pulp was 57.0 GB units.

Example 16 A paper furnish comprising a mixture of 50% groundwood, 50% sulfite pulp, rosin size, and papermakers alum at 50% pulp consistency was treated so as to incorporate on a dry basis, 1% sodium sulfoxylate formaldehyde. After moisture removal to 95% pulp consistency, the paper showed a brightness of 64.8 GB units whereas the unbleached paper showed a brightness of 57.1 GB units. Neither the treated or untreated paper absorbed 0.1 ml. of water in 15 minutes when tested by TAPPI standard testing method T432-m45, whereas the unsized paper showed complete absorption of moisture in 40 seconds.

Example 17 A paper furnish comprising a mixture of 50% groundwood, 50% sulfite pulp, a wet strength resin of the melamine formaldehyde type, and hydrochloric acid, at 50% pulp consistency, was treated so as to incorporate on a dry basis, 1% zinc sulfoxylate formaldehyde. After moisture removal to 95% pulp consistency, the paper showed a brightness of 65.9 GB units, whereas the un bleached paper showed a brightness of 56.7 GB units.

The treated paper had a wet tensile strength of 20.5 pounds per inch, and the unbleached paper had a wet tensile strength of 20 pounds per inch.

From the above examples and description of the invention, it will be seen that a procedure is described for brightening cellulosic pulps at pulp consistencies greater than about 20% by the employement as a bleachant of a water soluble salt of those oxy acids of sulfur wherein the valence of sulfur is less than 6 and the removal of moisture from the pulp substantially immediately thereafter. The amount of moisture removed is at least 50% of that present in the pulp at the time of chemical addition and may be as high as 98%. By such water removal 2. bleached pulp results of a consistency of from about to The amount of the reducing agent incorporated in the pulp will be dictated by the degree of brightening desired.

What is claimed is:

1. The method of brightening a cellulose pulp which comprises incorporating into a cellulose pulp having a pulp consistency of 20% to 60%, a water soluble salt of a reducing oxy acid of sulfur wherein the sulfur possesses a valence of less than 6 which is a reducing bleaching agent for cellulose pulps and in an amount to produce brightening of the pulp which is from about 0.1% to 1.0% on a dry pulp basis and substantially immediately thereafter heating the pulp to an elevated temperature to evaporate moisture therefrom rapidly and on the order of a few minutes until the pulp consistency has increased to about 95%.

2. The method of brightening a cellulose pulp which comprises incorporating into a cellulose pulp having a pulp consistency of 20% to 60%, a water soluble salt of a reducing oxy acid of sulfur which is a reducing bleaching agent for cellulose pulps selected from the group of reducing acids consisting of sulfurous acid, sulfoxylic acid and hydrosulfurous acid and in an amount to produce brightening of the pulp which is from about 0.1% to 1.0% on a dry pulp basis and substantially immediately thereafter heating the pulp to an elevated temperature to evaporate moisture therefrom rapidly and on the order of a few minutes until the pulp consistency has increased to about 95%.

References Cited in the file of this patent UNITED STATES PATENTS 1,409,799 Trostel Mar. 14, 1922 1,662,951 Bragg Mar. 20, 1928 2,071,305 Hirschkind Feb. 16, 1935 2,071,309 Hirschkind Feb. 16, 1935 OTHER REFERENCES Becco-B-ulletin No. 48, High Density Bleaching Processes, Buffalo Electra-Chem. 00., Buffalo, N.Y., 1953, (20 pp.; 11 dwgs.). 

1. THE METHOD OF BRIGHTENEING A CELLULOSE PULP WHICH COMPRISES INCORPORATING INTO A CELLULOSE PULP HAVING A PULP CONSISTENCY OF 20% TO 60%, A WATER SOLUBLE SALT OF A REDUCING OXY ACID OF SULFUR WHEREIN THE SULFUR POSSESSES A VALENCE OF LESS THAN 6 WHICHIS A REDUCING BLEACHING AGENT FOR CELLULOSE PULPS AND IN AN AMOUNT OF PRODUCE BRIGHTENING OF THE PULP WHICH IS FROM ABOUT 0.1% TO 1.0% ON A DRY PULP BASIS AND SUBSTANTIALLY IMMEDIATELY THEREAFTER HEATING THE PULP TO AN ELEVATED TEMPERATURE TO EVAPORATE MOISTURE THEREFROM RAPIDLY AND ON THE ORDER OF A FEW MINUTES UNTIL THE PULP CONSISTENCY HAS INCREASED TO ABOUT 95%. 